1. Field of the Invention
The present invention relates to a pneumatic radial tire in which excellent stability and controllability are provided and a large amount of road noise can be reduced
2. Description of the Related Art
In recent years, since requirements for the sophistication of vehicles have been increasing, especially passenger cars, improvements of riding qualities have been proceeding. Therefore, as requirements for a tire, reduction of noise caused by a tire and higher riding qualities through a tire are needed.
It is especially desired that a large amount of noise which is caused at the interior of a vehicle can be reduced. In this case, improvement requirements for so-called "road noise" that is one of the above-described noises have been extremely increasing. Namely, road noise is caused by the tire during travel of a vehicle picking up irregularities on a road surface. The resultant vibration is conveyed to the interior of a vehicle, thereby causing the air to vibrate in the interior of the vehicle.
Further, simultaneously with the sophistication of vehicles, speed-up and/or power-up of vehicles also occurs. Accordingly, a tire for which only problems in riding qualities and road noises have been solved in the same manner as a conventional tire cannot comply with the aforementioned requirements. As a result, it is necessary to maintain a tire to a high level of speed resistance, uniformity performance, stability and controllability, and rolling resistance.
In a conventionally known method of reducing road noise, a method can be provided in which the entire portion or the widthwise direction end portions of a crossing belt layer is/are held by a reinforcing layer which is made of rubber-coated cords such as nylon cords which are disposed in the circumferential direction of a tire so that the rigidity of the aforementioned belt layer in the circumferential direction thereof can be reinforced.
In accordance with this method, a ribbon-shaped strip is formed by rubber-coating a nylon cord or a plurality of nylon cords which are disposed in parallel to each other, and is wound around a tire substantially in the circumferential direction thereof. A green tire to which a tread member unvulcanized is attached is vulcanized on the above resulting tire so that a tire as a product is manufactured.
However, a problem arises in that, because nylon cords generally show Young's modulus which is relatively low at the range of temperature depending upon the use conditions of a fire (a large elongation 4.2% was resulted under load of 1.4 g/d at the temperature of 50.+-.5.degree. C.), it is impossible to improve road noise to the desired level. Moreover, since nylon cords tend to generate a flat spot on a fire, especially in winter, when a vehicle runs after having been parked for a long time, abnormal vibration may be caused.
On the other hand, in the case of aromatic series polyamide fiber (alamide fiber from Dupont Co., Ltd.), since Young's modulus is high even at the vulcanized temperature of 170.+-.5.degree. C. (a small elongation 0.8% was resulted under load of 0.7 g/d at the temperature of 170.+-.5.degree. C.), when cords are disposed in parallel to each other in the circumferential direction of a tire, they are not elongated during the inflation and vulcanization of a tire. Accordingly, the angle formed between the cords and a crossing belt layer (especially, a belt layer which is made of steel cords) which is positioned inwardly in the radial direction of a tire, i.e., the angle formed at the time when a green tire is inflated due to an internal vulcanization pressure, can be prevented from being changed so that desirable angle changes cannot be provided. Further, it is difficult to provide an essential amount of gauge for the rubber between a belt reinforcing layer and a belt layer so that the upper and lower cords are kept in close contact with each other, thereby causing a separation failure to the tire. In an excessive case, the belt reinforcing layer and the belt layer are displaced from each other, thereby affecting the uniformity of a tire adversely. This causes problems in noise and stability and controllability of a tire.